Hi Michal, On Fri, Jun 28, 2013 at 02:24:12PM +0200, Michal Hocko wrote: > On Fri 28-06-13 08:54:35, Minchan Kim wrote: > > Hello Michal, > > > > On Thu, Jun 27, 2013 at 06:11:03PM +0200, Michal Hocko wrote: > > > On Fri 28-06-13 00:35:28, Minchan Kim wrote: > > > > Hi Michal, > > > > > > > > On Thu, Jun 27, 2013 at 11:37:21AM +0200, Michal Hocko wrote: > > > > > On Thu 27-06-13 15:12:10, Hyunhee Kim wrote: > > > > > > In vmpressure, the pressure level is calculated based on the ratio > > > > > > of how many pages were scanned vs. reclaimed in a given time window. > > > > > > However, there is a possibility that "scanned < reclaimed" in such a > > > > > > case, when reclaiming ends by fatal signal in shrink_inactive_list. > > > > > > So, with this patch, we just return "low" level when "scanned < reclaimed" > > > > > > happens not to have userland miss reclaim activity. > > > > > > > > > > Hmm, fatal signal pending on kswapd doesn't make sense to me so it has > > > > > to be a direct reclaim path. Does it really make sense to signal LOW > > > > > when there is probably a big memory pressure and somebody is killing the > > > > > current allocator? > > > > > > > > So, do you want to trigger critical instead of low? > > > > > > > > Now, current is going to die so we can expect shortly we can get a amount > > > > of memory, normally. > > > > > > And also consider that this is per-memcg interface. And so it is even > > > more complicated. If a task dies then there is _no_ guarantee that there > > > will be an uncharge in that group (task could have been migrated to that > > > group so the memory belongs to somebody else). > > > > Good point and that's one of the reason I hate memcg for just using > > vmpressure. > > Well, the very same problem is present in the memcg OOM as well. oom > score calculation is not memcg aware wrt charges. > > > Let's think over it. One of the very avaialbe scenario > > which userland could do when notified from vmpressure is that manager > > process sends signal for others to release own cached memory. > > Assuming those processes are in the same memcg, right? > > > If we use vmpressure without move_charge_at_immigrate in multiple memcg > > group, it would be a disaster. But if we use move_charge_at_immigrate, > > we will see long stall easily so it's not an option, either. > > I am not sure I am following you here. Could you be more specific what > is the actual problem? > From my POV, a manager can see a memory pressure, it notifies others in > the same memcg and they will release their caches. With > move_charge_at_immigrate == 0 some of those might release a memory in > other group but somebody must be using memory from the currently > signaled group, right? My concern is that manager process can send a signal to a process A in same group but unfortunately, process A would release a memory in other group so manager process can send a signal to a process B in same group but unfortunately, process B would release a memory in other group so manger process can ... ... ... ... in same group and at last, process Z would release a memory in same group but we release all of cached from A-Y process. :( > > > So, IMO, it's not a good idea to use vmpressure with no-root memcg so > > it could raise the question again "why vmpressure is part of memcg". > > Maybe I do not see the problem correctly, but making vmpressure memcg > aware was a good idea. It is something like userspace pre-oom handling. I don't say that memcg-aware is bad. Surely it's good thing but it's not good that we must enable memcg for just using memory notifier globally. Even above problem would make memcg-vmpressure complicated and memory reclaim behavior change compared to long history well-made global page reclaim. I claim we should be able to use vmpressure without memcg as well as memcg. > > > I really didn't want it. :( > [...] > -- > Michal Hocko > SUSE Labs -- Kind regards, Minchan Kim -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>